1. Field of the Invention
This invention relates to safety systems for nuclear reactors. More specifically, this invention is directed to the override of nuclear reactor control with the purpose of insuring that minimum conditions are present at all times in order to insure the adequate operation of the nuclear reactor safety system.
2. Description of the Prior Art
The theory and operation of nuclear reactors is now well known. Quite briefly, a fissionable atom such as U.sup.233, U.sup.235, or Pu.sup.239 absorbs the neutron in its nucleus and undergoes a nuclear fission. This produces, on the average, two fission products of lower atomic weight and great kinetic energy, and several high energy fission neutrons. The kinetic energy of the fission products is quickly dissipated as heat in the nuclear fuel. A small fraction of the fission products consists of highly radioactive elements. In order to prevent the release of these radioactive fission products from the reactor fuel, the fuel is clad or enclosed in sealed cans. Accordingly, the operation of the nuclear reactor is limited by the temperature which the fuel element cladding materials will tolerate without failure. In order to adequately protect the reactor core against excessive temperatures is it necessary to provide a protection system which monitors various parameters of the reactor and which automatically takes control of the reactor and shuts the reactor down when dangerous conditions are approached. The critical parameters which have an effect on core temperatures include coolant temperature, coolant pressure, total reactor power, local power and coolant flow rates.
It is conventional in the art of reactor control to continuously monitor each of these parameters. The continuously monitored parametric values are then compared to independently pre-established high and low limits. One such prior art protection system which monitors only power is disclosed in U.S. Pat. No. 3,565,760 entitled "Nuclear Reactor Power Monitor System." This prior art patent proposes a way to monitor the bulk thermal power level and to scram the reactor when the bulk thermal power level or local power level is unacceptably high. Such reactor protection systems, as the one described in the above mentioned patent, limit themselves to the separate consideration of each parameter rather than addressing themselves to the plurality of parameters which are functionally interrelated to determine a core operating limit. Such prior art reactor protection systems which monitor only one of the multiplicity of pertinent parameters must or should be supplemented by a multiplicity of independent protection systems which independently monitor the other parameters. Thus, it is a characteristic of such prior art reactor protection systems to provide a multiplicity of separate systems that independently establish operating envelopes for each of the parameters but ignore the functional interrelationship between the parameters. The nuclear reactor power systems which are operated with such independent protection systems are subject to economic penalties imposed by unduly conservative safety system designs, since they are incapable of taking advantage of the functional interrelationship of the pertinent parameters.
In order to avoid the conservatism inherent in the parameter by parameter approach of these typical prior art protection systems, other protection systems have been developed to address themselves, at least partially, to the functional interdependence of the parameters of interest. Thus, a multiple parameter thermal margin trip system is disclosed in U.S. Pat. No. 3,791,922 issued on Feb. 12, 1974 and assigned to the assignee of the present invention. This thermal margin protection system utilizes the functional interdependence of the parameters of reactor coolant temperature, reactor coolant pressure and reactor power. In order for the multiple parameter protection systems to provide the protection function intended by their designers, there must be provided at least one additional independent means to assure that the parameters not addressed by the multiple parameter systems are maintained at either constant values or within assumed envelopes of values. For example, one of the requirements is that the reactor power distribution by maintained within acceptable bounds.
It is well understood that control rod insertion, as well as other phenomena, has an effect on reactor power distribution. In designing the reactor and the reactor control systems, the reactor designers establish allowable control rod insertion patterns and degrees of insertion for each power level. It has become apparent that at lower power levels certain control rod insertion patterns and power distributions are permissible that are not permissible at higher power levels. Thus, a control system and method are required which permit certain control rod patterns and power levels at lower power but which do not permit the same patterns and insertions at higher levels. The required system is complicated by the fact that power is dependent on variables other than control rod insertion, such as xenon poison, boron concentration in the coolant, coolant density and coolant void fraction. Accordingly, the present invention is intended to provide a system and a method for helping to assure that the power distribution is maintained within acceptable bounds at all times.